Centrifugal compressor hub containment assembly

ABSTRACT

To contain high energy fragments generated by a hub burst, a compressor containment assembly in a gas turbine engine includes a diffuser housing having an arch portion that defines a channel which traps the high energy fragments and an axial facing U-shaped which imparts flexibility. The assembly also includes a concave back shroud having a series of grooves that create resistance for the fragments as they slide along its surface. At its radial outer end the back shroud has an annular rim that absorbs some of the energy and delays the shearing of bolts. A bumper is disposed between the back shroud and a turbine wheel to limit the deflection of the back shroud during a hub burst.

TECHNICAL FIELD

This invention relates generally to centrifugal compressors for gasturbine engines, and in particular to a containment assembly forcontaining within the engine, high energy fragments produced duringcompressor disintegration.

BACKGROUND OF THE INVENTION

Small gas turbine engines, often referred to as Auxiliary Power Units,are mounted aboard many types of aircraft to supply pressurized air forenvironmental control systems and main engine starting as well as shafthorsepower to drive accessories such as electric generators or hydraulicpumps. These engines include in flow series arrangement of a compressor,a combustor, and a turbine. The turbine drives the compressor through arotating shaft. To keep these engine lightweight and small withoutsacrificing power it is necessary to operate the compressor at very highspeeds, typically in excess of 40,000 rpm. Though a rare occurrence,these compressors sometimes disintegrate spewing out high energyfragments. Should these fragments escape from the engine they canseriously damage the aircraft. Accordingly, engine designers are alwaysstriving to develop better containment assemblies for containing thehigh energy fragments.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a compressor hubcontainment assembly that can contain the high energy fragmentsgenerated during a hub burst.

The present invention achieves the above-stated objectives by providinga containment assembly that includes a diffuser housing having an archportion that defines a channel which traps the high energy fragments andan axial facing U-shaped which imparts flexibility. The assembly alsoincludes a concave back shroud having a series of grooves that createresistance for the fragments as they slide along its surface. At itsradial outer end the back shroud has an annular rim that absorbs some ofthe energy and delays the shearing of bolts. A bumper is disposedbetween the back shroud and a turbine wheel to limit the deflection ofthe back shroud during a hub burst.

These and other objects, features and advantages of the presentinvention, are specifically set forth in, or will become apparent from,the following detailed description of a preferred embodiment of theinvention then read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a portion of a gas turbine enginewith a centrifugal compressor and a containment assembly contemplated bythe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a cross-section of a gas turbine engine generally denotedby the reference numeral 10 in which the engine components areaxisymmetric about the engine centerline 8. The gas turbine engine 10 iscomprised in flow series arrangement of a centrifugal compressor 20, acombustor 80, and a turbine 90 operatively coupled to the compressor 20via a rotatable shaft 12. Air enters the engine 10 through an inlet 14,is pressurized in the compressor 20, and then mixed with fuel andignited in the combustor 80 to form a hot, pressurized gas. This gas isthen expanded across the turbine 90 to supply the work to drive thecompressor 20 and any other accessories that are mounted to the engine10.

The compressor 20 includes an centrifugal hub 22 which at one axial end19 is splined to the shaft 12. At its opposite axial end 18, a curviccoupling 96 couples the hub 22 to a labyrinth seal 92 which is mountedfor rotation to the shaft 12. Extending from the hub 22 are a pluralityof circumferentially spaced apart impeller blades 24 and a plurality ofsplitter blades 26. Each of the splitter blades 26 is disposed betweentwo of the impeller blades 24. The hub 22 and blades 24 and 26, arecollectively referred to as an impeller. The impeller is disposed withinthe engine 10 between a front shroud 28, and a back shroud 50. The frontshroud 28 is slightly spaced from the outer edges of the blades 24 and26, and in combination with the hub 22 defines an axially facing annularimpeller inlet 30 and radially outward facing impeller outlet 32.Upstream of the inlet 30 is an annular inlet plenum 31.

Circumscribing the outlet 32 are an outer annular wall 34 and a innerannular wall 36. The walls 34 and 36 are axially spaced apart and definean annular diffuser 38 which extends to the combustor 80. A plurality ofcircumferentially spaced diffuser vanes 39 are disposed within thediffuser 38 between the walls 34 and 36. At its radially outer end, thefront shroud 28 has a curved portion 27 that is opposite the outerannular wall 34 to define an annular channel 25 therebetween. The frontand back shrouds 28, 50, and the inner and outer walls 34,36 are coupledto a diffuser housing 40 by bolts 41.

The diffuser housing 40 is comprised of an annular, axially facingU-shaped portion 42 having a length-to-height ratio of about 2 to 1. TheU-shaped portion 42 is welded to, or attached by any other means, to aradially extending arcuate portion 46. The arch portion 46 defines aradially facing circumferential channel 48. The channel 48 should bemade as deep as available space permits without affecting the structuralintegrity of the arch portion 46. At its downstream end the arch portion46 is bolted to a combustor housing 82. The U-shaped portion 42 is madefrom a material, such as Inconel 625, having a larger elongationproperty than the material, such as Inconel 718, from which the archportion 46 is made.

The back surface of the inner wall 36 has an annular slot 37. At itsradial outer end the back shroud 50 has two radial spaced apart annularrims 52,54 which are inserted into the slot 37. The outer of these rims52, referred to as a pilot, contacts the surface of the slot 37 and isused to position the back shroud 50 within the engine 10. A 0.020 inchgap separates the inner 54 from the surface of the slot 37. Preferably,the rim 54 should be twice as thick as rim 52. Alternatively, the rims52 and 54 can be combined in a single rim. Moving inwards from the rim54, the back shroud 50 has a slightly concave middle portion 56. Themiddle portion is contoured to match the curvature and shape of the backsurface 23 of the hub 22. At its radially inner end 58, the back shroud50 has a plurality of rounded grooves 60. Attached to the inner end 58is a structure 62 having an end 64 placed within 0.25 inches of theturbine wheel 94. The end 64 should be as close as possible, withouttouching, to the turbine wheel 94, but at least within 0.50 inches ofthe turbine wheel. The structure 62 is referred to as a bumper becauseduring a hub burst it contacts the turbine wheel 94 preventing the backshroud 50 from deflecting away from the hub 22. In the preferredembodiment, the bumper 62 is part of the labyrinth seal 92 but has athickness at least twice that of conventional seals. Alternatively, thebumper 62 may be any structure disposed between the back shroud 50 andthe turbine wheel 94 that limits the deflection of the back shroudduring a hub burst.

In the event of such a burst, high energy fragments are channeled to thediffuser by the front and back shrouds 28, 50. The back shroud has anumber of novel features which act to contain these fragments. Thegrooves 60 create resistance for the fragments as they slide along thesurface of the shroud, and also grind off portions of the curviccoupling 96 which because of their sharpness are very damaging. Themiddle portion 56, due to its shape, also creates resistance for thefragments and may even trap some of them. The force generated by theburst pushes the wall 36 into contact with the rim 54 which absorbs someof this energy and delays the shearing of the bolts 41.

The diffuser housing 40 also has a number of novel features designed tocontain these high energy fragments. Fragments are caught by the channel48 and swirl around within the channel. The channel also prevents thesefragments from entering the combustor where the casings are usuallythinner. Under the force of the burst, the U-shaped portion 42 stretchesand deforms axially thereby preventing the breaking or tearing of thediffuser housing 40. The U-shaped portion 42 also traps some of thefragments.

The combination of back shroud 50 and diffuser housing 40 results in acontainment system that can contain the high energy fragments associatedwith a hub burst. By containing these fragments damage to the aircraftis avoided and flight safety is not jeopardized.

Various modifications and alterations to the above described embodimentwill be apparent to those skilled in the art. Accordingly, thisdescription of the invention should be considered exemplary in natureand not as limiting to the scope and spirit of the invention as setforth in the following claims.

What is claimed is:
 1. A gas turbine engine comprising:a turbine mountedto a shaft defining an axial direction; a combustor in fluidcommunication with said turbine; an impeller mounted to said shaft andhaving an axial inlet and a radial outlet, said impeller being disposedbetween a front shroud and a back shroud; an inner and outer annularwall circumscribing said radial outlet, said walls spaced apart axiallyto define therebetween a diffuser; a housing extending radially andaxially from said outer annular wall so as to circumscribe said annularwalls and said shrouds, said housing having a radially extending archportion that defines a circumferential channel; and said back shroudhaving a radially inner end with a plurality of grooves and a middleportion extending from said inner end, said middle portion having ashape similar to that of the back surface of said impeller.
 2. The gasturbine engine of claim 1 wherein said housing further includes anaxially facing U-shaped portion.
 3. The gas turbine engine of claim 2wherein said U-shaped portion is made from a material having a largerelongation property than the material from which the arch portion ismade.
 4. A centrifugal compressor comprising:an impeller disposedbetween an annular front shroud and an annular back shroud; a diffusercircumscribing said impeller; a housing circumscribing said diffuser andsaid shrouds, said housing having an arch portion that defines aradially facing circumferential channel and also having an axiallyfacing U-shaped portion; and a bumper means attached to the back shroudfor limiting the deflection of the back shroud during a hub burst. 5.The compressor of claim 4 wherein said back shroud includes a radiallyinner end having plurality of grooves, a middle portion extending fromsaid inner end and having a shape similar to that of the back surface ofsaid impeller.
 6. The compressor of claim 4 wherein said diffuser has awall with an annular slot for receiving at least one annular rimextending from said back shroud.
 7. The compressor of claim 4 whereinthe U-shaped portion is made from a material having a larger elongationproperty than the material from which the arch portion is made.